Field of the Invention
The invention relates to a hearing instrument, in particular a hearing aid.
Description of Related Art
Static or adaptive beamforming is a beneficial technique available in a hearing aid to help the wearer in challenging listening situations. Typically beamforming is achieved electronically by combining the signals from two omni-directional microphones (which are sensitive to acoustic pressure) or by using a single-membrane directional microphone having two sound ports. EP 0 652 686 discloses several variants of adaptive microphone arrays and methods of processing their signals.
Beamforming based on two omni-directional microphones is based on the directionally dependent phase difference between the two microphones and assumes that they are identical in magnitude and phase response. This feature has the disadvantage that the signal combination is sensitively dependent on the characteristics of the two microphones, which in reality are unavoidably slightly different due to manufacturing tolerances. For example, the tension of the microphone membranes or the size and geometry of an opening for the static pressure equalization may slightly vary from microphone to microphone. This requires a delicate post manufacturing adjustment process or adaptive matching during operational use, and brings about a residual inaccuracy. Overall, the matching requirement is a substantial obstacle in further product development and advancement.
In addition to adaptive beamforming, the prior art also teaches hearing instruments that can be switched between an omnidirectional mode in which the processed sound signal is taken from an omnidirectional microphone and a directional mode in which a directional microphone, such as a pressure gradient microphone, is used. CH 533 408, U.S. Pat. No. 5,808,147 and EP 2 107 823 teach examples of microphone arrangements in which a pressure microphone (omnidirectional microphone) and a pressure gradient or hypercardioid microphone (directional microphone) are integrated in a common casing. Solutions with switchable directivity between omni and a given pre-determined directivity require a manual or signal-dependent switching mechanism and cannot offer the full benefit of an adaptive beamformer.